1. Field of the Invention
The present invention relates to a zoom lens system, more particularly to a zoom lens system suitably used as a taking lens system in lens shutter cameras.
2. Description of the Prior Art
It is required that a zoom lens system used as a taking lens system in lens shutter cameras be made as compact and low-cost as possible, without diminishing their optical performance. As such lens system, various zoom lens systems that consist of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power have been proposed.
Zoom lens systems consisting of two lens groups, however, are defective in that it is not possible, especially at the wide-angle end, to properly correct magnification chromatic aberration and distortion at the same time as securing sufficient lens-backs required in a taking lens system for cameras.
Recently, even in the zoom lens system consisting of two lens groups, use of single lenses made of plastic is preferred nowadays in consideration of cost reduction. It is not possible, however, to obtain sufficient optical performance by simply aking such lens-system-constituting single lenses using lastic. This is because ambient temperature variation causes ariation in the shape and refractive power of the plastic lenses, with the result that the focal point of the whole system varies. To solve this problem, U.S. Pat. No. 5,368,321 and Japanese Laid-Open Patent Application No. H5-113537 propose a two-lens-group zoom lens system in which the variation in focal point of a plastic lens is corrected. This type of zoom lens system, however, is defective in that the plastic lens in it is far from serving well enough to correct aberration, because the plastic lens is designed to have almost no refractive power.
Moreover, U.S. Pat. No. 5,327,290 proposes a zoom lens system consisting of a front lens unit composed of one positive lens, and a rear lens unit composed of one negative lens.
Moreover, U.S. Pat. No. 5,218,478 proposes a zoom lens system consisting of a first lens unit composed of two single lenses and having a positive refractive power as a whole, and a second lens unit composed of one negative lens.
The zoom lens systems disclosed in U.S. Pat. Nos. 5,327,290 and 5,218,478, however, are defective in that chromatic aberration cannot be corrected properly.